Abstract: The partial penetrating crack in the weld fusion zone of the measurement tube branch pipe and a root valve pipe in the main system secondary loop flow was analyzed in Qinshan Nuclear Power Plant. The macro and scanning electron microscope（SEM） on the end face was observed. Element component analysis was carried out on the different area of the end face. The microstructure and grain size analysis was carried out on the joint section. Joint finite element analysis combined with the pipeline running status came to the conclusion that the crack initiation is caused by the fatigue. The impulse by the change of the fluid flow state causes the valve to vibrate when the fluid flows through the valve. This makes the valve to bear a certain frequency of pull-pull stress. Because there is a large internal pressure, a great stress concentration is produced on the internal wall of the root valve. Fatigue cracks are generated at the root position of the welding seam. After the crack is produced, it is developed into the corrosion fatigue crack under the action of internal fluids.